This invention relates to the use of mixtures of oxidizers in minimum-smoke crosslinked double base (XLDB) propellants whose particle sizes can be adjusted for the specific purpose of dramatically increasing the burning rate of the propellant.
High performance solid rocket propellants with a minimum visible signature, or minimum smoke, can be manufactured by combining solid oxidizer and binders that contain only the elements carbon, hydrogen, nitrogen, and oxygen. Oxidizer include but are not limited to cyclotrimethylenetrinitramine (RDX), cyclotetramethylenetetranitramine (HMX), pentaerythritol tetranitrate (PETN) and ammonium nitrate (AN), with RDX and HMX being the most common oxidizers used because of their increased performance when compared to other oxidizers. Binders consist of mixtures of polymers that can be crosslinked during cure and nitro and nitrate ester plasticizers. Typical polymers include but are not limited to poly(ethylene glycol adipate) (PGA), polycaprolactone (PCP), and poly(ethylene glycol) (PEG) with hydroxyl functionality between two and three. These polymers are cured with a combination of (1) polyfunctional alcohols such as nitrocellulose (12.2% nitrogen) (NC), butane triols, and hexane triols and (2) polyfunctional isocyanates such as hexamethylene diisocyanate (HDI), isophorone diisocyanate (IPDI), toluene diisocyanate (TDI) and aliphatic polyisocyanates such as Mobay N-100.RTM. with an isocyanate functionality between 3 and 4. Typical nitro and nitrate ester plasticizers include but are not limited to one or more liquids such as a 1/1 mixture of bis-dinitropropyl acetyl (BDNPA) and bis-dinitropropyl formal (BDNPF), nitroglycerin (NG), butane triol trinitrate (BTTN), trimethylol ethane trinitrate (TMETN) and tri(ethylene glycol)dinitrate (TEGDN), with NG and BTTN being the most common plasticizers used because of their increased performance when compared to other plasticizers. Small quantities of stabilizers are added to increase the useful life, or shelf life, of these propellants. Typical stabilizers include but are not limited to 2-nitrodiphenylamine (DNPA), N-methyl-p-nitroaniline (MNA), and 1,3-bis(N-methyl-phenyl urethane)benzene (BMUB).
It is very difficult to tailor the burning rate of minimum smoke XLDB propellants. Varying the RDX or HMX particle size has very little effect on propellant particle burning rates. Small particle size, 10 to 20 microns weight-medium-diameter, oxidizer is generally used because the resulting propellant will have better mechanical properties than a propellant with large oxidizer. Burning rate can be varied some by changing the binder energy. However, this technique is limited because this change will either reduce performance or reduce mechanical properties.
Propellant burning rates can be varied over the range of 0.2 in/sec to 0.5 in/sec at 1000 psi using the methods described above and by the addition of small quantities of lead and tin salts with carbon black. Typical lead and tin salts include but are not limited to lead citrate (PbCit), lead salicylate (PbSal), lead sebacate (PbSeb), lead oxides (PbO, Pb.sub.2 O.sub.3), tin citrate (SnCit), and lead stannate (PbSnO.sub.4) with PbCit being the most common lead salt used in minimum-smoke XLDB propellants. Generally less than three percent PbCit is employed and the amount of primary smoke generated by this lead in the rocket exhaust is minimal. The combustion characteristics of minimum-smoke propellants containing HMX are almost identical to propellants containing RDX.